Solid carbon dioxide pellets have been used as an abrasive to clean paint, coatings, or other materials from surfaces. Since solid carbon dioxide evaporates or sublimes under ambient conditions, there is no abrasive residue remaining from the pellets themselves after the cleaning operation. Thus solid carbon dioxide blast systems are especially preferred where large quantities of residue (e.g., sand from sandblasting operations) would be difficult to remove or where contamination from the residue would be undesirable.
Fong, U.S. Pat. No. 4,038,786 (Aug. 2, 1977) provided a solid carbon dioxide blast system using an air compressor to propel the carbon dioxide pellets. To reduce sublimation of the carbon dioxide pellets in the warm compressed air stream, Fong recommended that the dwell time of the carbon dioxide pellets in the carrier gas be kept to a minimum. Fong also recommended "cooling jackets" on the conduit leading from the compressor to the mixing chamber and on the carbon dioxide pellet hopper. Fong et al., U.S. Pat. No. 4,389,820 (June 28, 1983) provided another solid carbon dioxide blast system using vaporized carbon dioxide as the carrier gas. To prevent plugging up the system (i.e., clumping of the carbon dioxide pellets or formation of carbon dioxide snow), the carrier gas was heated to about 250.degree. to 275.degree. F.; the carrier gas temperature was never allowed to fall below 100.degree. F. With the carrier gas at these high temperatures, there will clearly be significant losses from the carbon dioxide pellets from sublimation.
Moore, U.S. Pat. No. 4,617,064 (Oct. 14, 1986) provided a solid carbon dioxide blast system using solid carbon dioxide pellets propelled with compressed air directly from an air compressor. Compressed air directly from a conventional air compressor will be relatively warm (i.e., 70.degree. to 100.degree. F. or higher). Moore used a rotary feed system to mix the compressed air and solid carbon dioxide pellets. The rotary feed system employed various seals to prevent the carrier gas from entering the carbon dioxide pellet storage container where, because of water vapor possibly present in the carrier gas, clumping of the carbon dioxide pellets might occur. The carbon dioxide pellet storage container was also pressurized with a positive pressure of carbon dioxide gas to minimize leakage of water vapor into the container.
More recently, Armstrong, U.S. Pat. No. 5,184,427 (Feb. 9, 1993) and Armstrong et al., U.S. Pat. No. 5,365,699 (Nov. 22, 1994) provided a carbon dioxide blast system using only cryogen (e.g., vaporized liquid nitrogen) as the carrier gas. The air compressor normally used in the carbon dioxide blast systems was completely eliminated in this design. The carrier gas was generated by vaporizing the cryogen (preferably using an ambient air vaporizer). In this manner, the temperature of the carrier gas could be controlled between about -200.degree. F. and ambient. This carbon dioxide blast system used large quantities of cryogens. Moreover, liquid oxygen was sometimes required to be used in conjunction with the normal liquid nitrogen cryogen when the blast system was to be used on closed or confined areas. Workers in such closed or confined areas could still be exposed to significant danger if, for example, the liquid oxygen supply system failed or the oxygen levels fell below safe levels. For this reason, an oxygen alarm was recommended. In addition, liquid oxygen itself can present hazards, including the possibility of rapid oxidation and increased fire hazards.
It would be desirable, therefore, to provide a carbon dioxide blast system which minimizes and controls the sublimation of the solid carbon dioxide pellets. It would also be desirable to provide a carbon dioxide blast system which employs compressed air which can be cooled to a relatively low temperature, thereby minimizing the sublimation of the carbon dioxide pellets while at the same time allowing greater control of the hardness, size, density, impact forces, stripping rates, and stripping efficiencies of the carbon dioxide pellets. It would also be desirable to provide a carbon dioxide blast system which does not consume large quantities of cryogen and/or which does not require the use of liquid oxygen. It would also be desirable to provide a carbon dioxide blast system which significantly reduces the risks associated with using the system in closed or confined spaces. It would also be desirable to provide an improved method of pneumatically carrying carbon dioxide pellets which can be used in both blasting and non-blasting applications. The present invention provides such a carbon dioxide blast system and a pneumatically carried carbon dioxide pellet system.